The invention relates to a mask for EUV lithography, an EUV lithography system having such a mask and a method for optimising the imaging of a mask in an EUV lithography system.
Projection exposure systems for microlithography, referred to briefly as lithography systems below, generally comprise a light source, an illumination system which processes the light beams emitted by the light source to form illumination light, an object to be projected which is generally referred to as a reticle or mask, a projection objective which images an object field onto an image field and another object, on which the image is projected and which is generally referred to as the wafer or substrate. The mask or at least a portion of the mask is in the object field and the wafer or at least a portion of the wafer is in the image field of the projection objective.
If the mask is completely in the range of the object field and the wafer is illuminated without relative movement of the wafer and image field, the lithography system is generally referred to as a wafer stepper. If only a portion of the mask is in the region of the object field and the wafer is illuminated during relative movement of the wafer and image field, the lithography system is generally referred to as a wafer scanner. The spatial dimension defined by the relative movement of the reticle and wafer is generally referred to as the scanning direction.
The integration density of a lithography system that can ultimately be achieved on the wafer is substantially dependent on the following parameters: (a) depth of focus (DOF) of the objective lens, (b) image-side numerical aperture (NA) and (c) wavelength of the illumination light. For reliable operation of a lithography system, it is necessary to ensure a so-called process aperture which is as large as possible and which comprises possible focus variation (FV) and variation of the amount of illumination light for a desired critical dimension (CD), that is to say, the smallest structural width occurring on the wafer, and a given numerical aperture (NA). In order to further reduce the CD, the development in EUV lithography systems is generally towards increasing numerical apertures NA. In the following description, the term “critical dimension” does not refer to the minimum structural width but will instead be used as a synonym for the line width or the structural width.
Periodic structures are decisively important for the integration density of the integrated switching circuit which is intended to be produced by means of the microlithographic illumination process. They are described by the pitch, that is to say, period length, and structural width. The structural width used on the wafer can be freely adjusted up to a given degree by the lacquer threshold of the resist to be illuminated. However, the smallest achievable pitch Pitchmin is given by the wavelength λ of the illumination light and the object-side numerical aperture NA of the objective lens. The following applies:Pitchmin˜λ(NA(1+σ))to coherent and incoherent illumination with a predetermined σ setting.
Currently, the best resolutions of pitch and CD are operated by so-called EUV lithography systems having a source of weak X-radiation (referred to as EUV/extreme ultraviolet in technical jargon) at a wavelength of the illumination light of approximately 13.5 nm. These are also referred to as EUV systems in technical jargon. The associated objective lenses which image the illuminated mask onto the wafer are catoptric objective lenses. These are typically operated with image-side numerical apertures of from 0.2 to 0.35 or above. Reference may be made to, for example, US2005 0088760A1 or US2008 0170310A1.
The mask to be imaged generally has a glass substrate, such as ULE™ or Zerodur®, which becomes highly reflective with light having a wavelength of 13.5 nm owing to a stack of dielectric layers, in particular alternating Mo and Si layers, and the mask structure which is in the form of a structured absorber layer on the layer stack is again defined by a structured chromium layer or by a structured layer of tantalum nitride (TaN) or other materials. The thickness of the structured absorber layer or the mask structure is typically more than 100 nm.
The structures on the mask to be imaged generally have two preferred directions. When the imaging qualities of a projection illumination system are assessed, therefore, a distinction is drawn at least between the maximum resolvable pitch of H (horizontal) and V (vertical) structures. In this instance, it should be hereinafter agreed that an H structure is intended to refer to a sequence of light-permeable and non-light-permeable regions of the mask structure, each individual region of those regions having its greater extent so as to be orthogonal relative to the scanning direction.
The illumination of the mask takes place with reflection in EUV lithography systems. Therefore, it is not possible to have telecentric illumination of the mask in non-obscured systems because otherwise the illumination system and the objective lens would be in the way. The chief ray angle (synonym: chief ray angle (CRA)) is the angle of the main beam of the illumination light relative to a notional orthogonal with respect to the object plane of the objective lens. In a projection exposure system as set out in US2005 0088760A1, a CRA of 6° is used with an image-side numerical aperture NA of 0.33. In a projection exposure system as set out in US2008 0170310A1, a CRA of 15° is used with an image-side numerical aperture NA of 0.5. Generally, the CRA used increases with the numerical aperture NA of the objective lens.
As set out in greater detail below, the CRA which is different from 0° results in shadows of the reflected illumination light owing to the extent of the mask structures orthogonally relative to the object plane of the objective lens. In this instance, this is a purely topographical effect of the mask which is determined by the geometric, three-dimensional arrangement of the illumination system, mask and objective lens. This is no longer negligible for EUV lithography systems particularly with a high NA and therefore a high CRA because the thickness of the structured layer (mask structure) is generally several wavelengths of the illumination light of 13.5 nm, and reference can consequently be made to shadowing of the mask structure.
Simple geometric observation makes it apparent that this shadowing appears more intensely for H structures than for V structures if it is assumed for the design of the illumination system and the objective lens that the incidence plane of the CRA on the mask is orthogonal to the extent of an individual structure of the H structures. The size of the difference of the structural widths of H and V structures on the wafer with the structural widths on the mask being assumed to be identical depends on the position thereof as an object point when viewed in the object plane of the objective lens, more specifically the azimuth angle at which the object point is orientated relative to an axis of a coordinate system which is in the object plane, which axis extends in the scanning direction. Since the CRA is measured in a plane perpendicularly relative to the object plane in which the axis of the coordinate system is located, the CRA appears to be rotated for object points which are not located on that axis, that is to say, which have an azimuth angle different from 0°.
H structures are generally imaged so as to be wider in dependence of their position in the object plane of the objective lens. There is further produced for H structures a displacement of the image which is dependent on the focal position of the mask and which corresponds to a field-dependent redirection of the wave front Z2, Z3 which can be corrected by correcting the focal position of the mask. The coefficients Z2, Z3 are Zernike coefficients whose indexing follows Fringe notation, cf. “Handbook of Optical Systems, Singer et al. (eds.), WileyVch, 2005”. If the entire wave front is analysed, field-dependent distortion terms Z2, Z3, focus variations Z4 and astigmatism Z5, Z6 result as aberrations. These are accompanied by wave front errors of higher orders, such as comas Z7, Z8 and secondary astigmatism Z12, Z13.
The explanation for the occurrence of those imaging errors which are also referred to below as “rigorous effects” is explained in greater detail, for example, in the article by J. Ruoff: “Impact of mask topography and multi-layer stack on high NA imaging of EUV masks”, Proc. SPIE 7823, September 2010. Rigorous effects are considered in the above-mentioned article. The rigorous effects depend on the structural widths, the material which defines the structures of the mask such as, for example, Cr, and the thickness of those structures in the direction of the beam path of the illumination light in the region of the mask.
In addition to the imaging errors which are produced owing to the shadowing or the rigorous effects of the mask structure and which become perceptible in particular in the form of a telecentricity error, the fact that the multi-layer coating has a reflectivity which varies over the incidence angle range of the radiation which strikes the mask also results in a variation of apodisation in the objective pupil which also results in a telecentricity error during imaging. However, those two contributions to the telecentricity error cannot be optimised separately from each other owing to the non-linearity of the rigorous effects to be considered.
In lithography systems, particularly in EUV lithography systems, there is a need for correcting the structural width and pitch-dependent aberrations of the wave front which are induced by the rigorous effects of the mask, and there is a particular need to correct the structural width and pitch-dependent astigmatism which is induced by the mask, the structural width and pitch-dependent distortion which is dependent on the position of the object point and the structural width and pitch-dependent focal position.